1. Field of the Invention
The present invention relates, in general, to an inorganic-organic copolymer using a polyvinylalcohol-silane coupling agent and its preparation. More particularly, the present invention relates to use of a coupling agent comprising a silane moiety containing an organic functional group capable of the introduction of organic compounds as well as an alkoxide substituent capable of the condensation with inorganic compounds in preparing an inorganic-organic copolymer which is free of phase separation as well as superior in resistance to heat and moisture and in mechanical properties.
2. Description of the Prior Art
When applied to polymer surfaces which have been treated to render them receptive to coatings, polyvinylalcohol, which is of high hydrophilicity, provides an oxygen impermeable barrier and is resistant to permeation by common organic solvents under anhydrous conditions. However, its high water solubility makes it difficult for polyvinylalcohol to conduct its own functions as a coating barrier on a polymer substrate. Poor water resistance of polyvinylalcohol restricts its use as a barrier coating to those few applications where nearly anhydrous conditions prevail. Moisture is known to cause damage on the coating barrier, an unpleasant feel to the touch, and deterioration in oxygen impermeability.
Attempts have been made in the past to overcome these problems. In fact, there have been known a number of methods for increasing the water resistance of polyvinylalcohol. Of them, combination methods of inorganic and organic materials are in active study.
Inorganic-organic combined materials are complexes in which inorganic materials are combined with organic materials in nano levels. When applied to polymer surfaces, these combined materials show improved physical properties without neither cracks, which are frequently found in inorganic materials, nor poor thermal stability and mechanical strength, which always shadows organic materials. However, such combined materials may feasibly suffer phase separation when the inorganic content is increased, which restricts their compositions to a narrow range.
In the present invention, the phase separation is overcome by use of a coupling agent comprising a silane moiety containing an organic functional group capable of the introduction of organic compounds as well as an alkoxide substituent capable of the condensation with inorganic compounds.
Conventional polymeric films for use in packaging can be referred to U.S. Pat. Nos. 5,604,042 and 5,512,338 in which polyurethane, polyethylene or polyethylene terephthalate films can be improved in oxygen impermeability by a coating of a solution obtained by the reaction between polyvinyl alcohol and melamine-formaldehyde. Improved as they are in oxygen barrier characteristics, the polymeric films are poor in moisture resistance. The reference patents further suggested a coating of cellulose or polyvinylidene chloride.
Improvements in the oxygen impermeability are found in U.S. Pat. Nos. 5,547,764, 5,496,649, 4,416,938 and 4,262,067. According to these references, the hydroxy group of polyvinyl alcohol is crosslinked with aldehyde by use of glyoxal and glutaric dialdehyde to better the oxygen barrier characteristics of the film. Although the hydroxy group of the polyvinyl alcohol reacts with aldehyde to form acetal, it plays no roles in the coupling between polyvinyl alcohol chains.
Korean Pat. Laid-Open Publication No. 95-23518 discloses a gas impermeable film which is improved in moisture resistance by use of polyvinyl alcohol and poly(meth)acrylic acid. The above-mentioned patents, which employ organic compounds only, however, show limited mechanical properties.
U.S. Pat. Nos. 3,773,776, 4,016,129, 4,478,909 and 5,134,021 describe anti-fogging films which recruit silica and polyvinyl alcohol. The prepared polyvinyl alcohol-silica compounds simply utilize hydrogen bonds. To improve anti-fogging characteristics, fluorine compounds and aluminum ligand precursors are used in combination.
PCT WO 94/07947 discloses that tetraalkoxy silane is reacted with polyvinyl alcohol in the presence of formic acid to form SiO2 bonds as well as to couple the hydroxy groups of the polyvinyl alcohol chains with the formic acid so as to improve toughness. This reference patent, however, explains a result of the reaction between tetraalkoxy silane and polyvinyl alcohol as the dispersion of polyvinyl alcohol in the inorganic network, instead of the formation of SiO2-polyvinyl alcohol linkages.
Commonly, the conventional composite materials suffer from disadvantages. For example, a slight phase separation occurs between inorganic materials and polymers, greatly deteriorating the transparency. Also, an excess of acid exists within the composite films, so that the organic materials are apt to be decomposed during a drying process.
It is an object of the present invention to resolve the problem encountered in prior arts and to provide an inorganic-organic copolymer which is free of phase separation as well as superior in resistance to heat and moisture and in mechanical properties.
It is another object of the present invention to provide a method for preparing such an inorganic-organic copolymer, using a silane coupling agent.